The development of microporous foams is the subject of substantial commercial interest. Such foams have found utility in various applications such as thermal, acoustic, electrical, and mechanical insulators; absorbent materials; filters; membranes; floor mats; toys; carriers for inks, dyes, lubricants, or lotions; and the like. References describing such uses and properties of foams include Oertel, G., Polyurethane Handbook, Hanser Publishers, Munich, 1985; and Gibson, L. J., Ashby, M. F., Cellular Solids, Structure and Properties, Pergamon Press, Oxford, 1988. The term "insulator" refers to any material which reduces the transfer of energy from one location to another. The term "absorbent" refers to materials which imbibe and hold or distribute fluids, usually liquids, an example being a sponge. The term "filter" refers to materials which pass a fluid, either gas or liquid, while retaining particulate matter suspended in the fluid by size exclusion or other means. Other uses for foams are generally obvious to one skilled in the art.
Open-celled foams prepared from High Internal Phase Emulsions (hereinafter referred to as "HIPEs") are particularly useful in a variety of applications including:
absorbent disposable articles: U.S. Pat. Nos. 5,331,015 (DesMarais et al.) issued Jul. 19, 1994; 5,260,345 (DesMarais et al.) issued Nov. 9, 1993; 5,268,224 (DesMarais et al.) issued Dec. 7, 1993; 5,632,737 (Stone et al.) issued May 27, 1997; 5,387,207 (Dyer et al.) issued Feb. 7, 1995; 5,786,395 (Stone et al.) Jul. 28, 1998; and 5,795,921 (Dyer et al.) issued Aug. 18, 1998; PA1 insulation e. g. (thermal, acoustic, mechanical): U.S. Pat. Nos. 5,770,634 (Dyer et al.) issued Jun. 23, 1998; 5,753,359 (Dyer et al.) issued May 19, 1998; and 5,633,291 (Dyer et al.) issued May 27, 1997; PA1 filtration: Bhumgara, Z. Filtration & Separation 1995, March, 245-251, Walsh et al. J. Aerosol Sci. 1996, 27, 5629-5630; and in published PCT application W/O 97/37745, published on Oct. 16, 1997, in the name of Shell Oil Co.; PA1 A relatively high molecular weight of the emulsifier hydrophobe is typically required in order to stabilize water-in-oil emulsions with the desired droplet size at very high internal phase ratios and at the temperatures required to effect cure of the polymer comprising the HIPE foam. PA1 The melting point of the emulsifier should be below the in-use temperature of the HIPE foam where the foam is intended for use in applications involving the rapid absorption of aqueous fluids. Emulsifiers with higher melting points that are suitable for the formation of high internal phase water-in-oil emulsions tend to be waxy in nature and do not typically produce foams which imbibe aqueous fluids rapidly. PA1 The emulsifier should not excessively plasticize the polymer comprising the HIPE foam. Typically, emulsifiers comprising highly branched hydrophobes tend to produce HIPE foams with relatively low resistance to compressive deformation. This is believed to be due to plasticization of the polymer by the branched hydrophobe and is particularly evident in low density HIPE foams that were formed from high water to oil ratio HIPEs. PA1 The emulsifier should be chemically stable during the storage and use of the HIPE foam prepared using such emulsifier. Any emulsifier remaining in the foam should not undergo any undesirable reactions or yield any undesirable chemical species. Emulsifiers with unsaturated hydrocarbon hydrophobes tend to be oxidatively unstable under normal storage conditions and can give rise to relatively low molecular weight aldehydes with characteristic unpleasant odor. The rate of oxidation and odor formation is exacerbated because the emulsifier is effectively spread over the high surface area of the HIPE foam. Exposure to high temperatures and/or ultraviolet light further accelerates oxidation.
and various other uses.
The cited patents and references above are incorporated herein by reference. The HIPE process provides facile control over the density, cell and pore size and distribution, proportion of cell struts to windows, and porosity in these foams.
The physical properties of the foam are governed by: (1) the properties of the polymer from which the foam is comprised, (2) the density of the foam, (3) the structure of the foam (i.e. the thickness, shape and aspect ratio of the polymer struts, cell size, pore size, pore size distribution, etc.), and (4) the surface properties of the foam (e.g., whether the surface of the foam is hydrophilic or hydrophobic). The emulsifier used to stabilize the HIPE can have a profound influence on such properties.
Without being bound by theory, a number of factors are believed to be important in determining the suitability of an emulsifier for producing HIPE foams with desirable physical characteristics.
In addition to the above criteria, the emulsifier should be relatively easy to produce in commercial quantities at a reasonable cost, and it should be safe for use in the intended application of the HIPE foam.
Various sorbitan esters and polyglycerol fatty esters have been used as emulsifiers for HIPE emulsions in absorbent foam applications, as exemplified in the aforementioned U.S. Pat. No. 5,387,207. This patent teaches the use of a commercial sorbitan ester which is a complex blend of surface active components, at least a portion of which comprises sorbitan monolaurate, along with diesters, higher molecular weight hydrophobes, isosorbide esters, and the like. While sorbitan monolaurate can be used to produce HIPEs and foams having desirable properties, such foams are limited to relatively high densities because of the low internal phase ratios achievable with this emulsifier. Sorbitan monolaurate is also typically limited to producing foams with relatively small average cell size. The non-sorbitan-monolaurate components further limit the internal phase ratios that are achievable. It will be recognized that commercial quantities of substantially pure sorbitan monoesters would be significantly more difficult to produce than the commercially available blend of materials and would thus be more expensive. Similarly, polyglycerol esters are also relatively difficult to produce in a substantially pure form. Without being bound by theory, it is believed that the molecular weight of the monolaurate hydrocarbon hydrophobe is too low to stabilize high internal phase water-in-oil emulsions with relatively large droplets of the discontinuous aqueous phase at the W:O ratios and temperatures required to cure the continuous external monomeric oil phase at a commercially satisfactory rate. In order to prepare foams with lower density and/or larger average cell size, the hydrocarbon hydrophobe should preferably have (on average) more than 14 carbon atoms, and more preferably more than 16 carbon atoms, while retaining a relatively low melting point for hydrophilicity, as described above.
Emulsifiers comprising saturated linear hydrocarbon hydrophobes with relatively high molecular weight, such as sorbitan monostearate or diglycerol monostearate may be used to produce HIPE foams with desirable cell sizes and with relatively high resistance to compression. However, owing to the relatively high melting points of these emulsifiers, such foams do not typically imbibe aqueous fluids rapidly under normal in-use temperatures (e.g. ambient and/or body temperatures).
One method of achieving a hydrocarbon hydrophobe with both relatively high molecular weight and relatively low melting point is to incorporate one or more cis C.dbd.C double bonds into the hydrocarbon chain. An example of a prior art emulsifier which functions very well in providing foams having desirable properties is diglycerol monooleate as discussed in commonly assigned U.S. Pat. No. 5,786,395 (Stone et al.) issued Jul. 28, 1998 the disclosure of which is incorporated herein by reference. Foams with desired average cell sizes and densities can be prepared using diglycerol monooleate. Such foams typically have good mechanical properties and can imbibe aqueous fluids rapidly under typical in-use conditions. However, this emulsifier is oxidatively unstable due to unsaturation in the oleate hydrophobe. This leads to malodor formation over time, as described above.
Another method of achieving a hydrocarbon hydrophobe with both relatively high molecular weight and relatively low melting point is to incorporate branching into the hydrocarbon moiety. The aforementioned U.S. Pat. No. 5,786,395 discusses the suitability of branched fatty hydrophobes. Although emulsifiers comprising such branched hydrophobes have been found to provide foams having desirable properties at relatively high densities, low density foams prepared with such emulsifiers (i.e. those prepared from water-in-oil emulsions with very high internal phase ratios) tend to have relatively low resistance to compressive deformation. Without being bound by theory, it is believed that such branched hydrophobes tend to plasticize the polymer comprising the foam excessively, thereby weakening the foam structure.
As can be seen the emulsifiers used by the art to stabilize HIPEs in the manufacture of HIPE-based foams all have properties that make them less than desirable.
Accordingly, it would be desirable to develop emulsifier materials suitable for stabilizing HIPEs, such that the foams produced from these HIPEs have all of the desirable properties of HIPE foams, including the desired density; structure (e.g. cell size and cell size distribution); mechanical properties (e.g. resistance to compressive deformation); fluid handling properties (e.g. rapid uptake of aqueous fluids); and chemical stability (e.g. resistance to degradation and/or odor formation). It would be still more desirable if such emulsifiers could provide such desirable properties at an economical cost.